Hello,

Newbie here. Need opinions on the best way to control the speed of a 3V dc motor. I have tried a couple of pots but the control is to condensed...in other words all the control is happening within a quarter turn. The range of RPMs I am looking for is 15-40, so pretty slow. The motor I have is Radio Shack's # 2730223

This unit looks to do exactly what I want but it looks like a servo in the beginning of the video. Should I go that route? Or do I keep the motor I have and use gears to knock down the RPMs? Pretty confused.

In the end, I'd like this to be battery powered and fit in a small project box.

 

Your pot is too high in resistance. You need one that is likely no more than 100Ω to 500Ω full resistance.

 

The motor is around 2700rpm/volt so if looking for 15-40 rpm with that motor you are probably going to need another method or a different motor or even gearing?
The video appears to be using a RC servo, but only two leads instead of the customary 3 for RC PWM but usual for the linear actuator?

Max.

 

Max - I don't think that's a servo; I think it's a geared motor. The video identifies it as a micro gear motor (at the beginning, they list the part numbers)

Detail about the device is here

 

Probably identical to a RC servo without the feedback etc maybe?

Max.

 

Max - I don't think that's a servo; I think it's a geared motor. The video identifies it as a micro gear motor (at the beginning, they list the part numbers)

Detail about the device is here

Probably identical to a RC servo without the feedback etc maybe?

Max.

I bought a servo from Radio Shack but didnt know how to mod it for full rotation. I took it back but have found a few video with mods on how to do this. Should I go this route?

 

Hello,

Newbie here. Need opinions on the best way to control the speed of a 3V dc motor. I have tried a couple of pots but the control is to condensed...in other words all the control is happening within a quarter turn. The range of RPMs I am looking for is 15-40, so pretty slow. The motor I have is Radio Shack's # 2730223

This unit looks to do exactly what I want but it looks like a servo in the beginning of the video. Should I go that route? Or do I keep the motor I have and use gears to knock down the RPMs? Pretty confused.

In the end, I'd like this to be battery powered and fit in a small project box.

Look at your pot. If it has the resistance values and a letter "A", then you have a log (audio) pot and the response will be compressed. Try one with a "B" for a linear response. In any case, you will have a fairly jittery response at those RPMs with a simple resistive control and almost no 'power' (torque).

You can try a PWM motor control for slower with better torque and speed control at low rpms.

 

If you're confident you can do the mod then that would provide a simple way of getting a geared motor relatively cheaply. However, standard servos are pretty nippy, so you will probably still need some way of reducing its speed. A suitable pot can reduce speed, but torque will also be reduced. PWM can reduce speed but still give good torque.

 

I guess I'm repeating what other people said, but...trying to modulate a very cheap, 8000 RPM motor down to 15 RPM is like trying to make a Swiss watch powered by a mouse on meth. You really need to start with something that has a chance of working.

 

I bought a servo from Radio Shack but didnt know how to mod it for full rotation. I took it back but have found a few video with mods on how to do this. Should I go this route?

A RC servo has a single turn pot on it for feedback so you would need to take it apart and modify for direct DC to the motor instead of the internal single turn PWM control.
The motor controller in your link appears to be a PWM version, there are many on ebay for around $4.00.
Max.

 

Maybe something like this?
http://www.ebay.com/itm/DC3-6V-Shor...122?pt=LH_DefaultDomain_0&hash=item5b0a60f46a

 

I ended up buying a RioRand mini 12V DC 60rpm High Torque Gear Box Electric motor and a 12V-40V 10A PWM DC Motor Speed controller. These two combined do exactly what I need. However the PWM prevents reverse voltage and I want the motor to run in reverse at times as well. So I added a DPDT switch after the PWM and before the motor and this works perfect. My question is, will reversing the voltage hurt the motor? Short test indicate that it seems to work fine, but not sure what might happen with longer run times.

 

I ended up buying a RioRand mini 12V DC 60rpm High Torque Gear Box Electric motor and a 12V-40V 10A PWM DC Motor Speed controller. These two combined do exactly what I need. However the PWM prevents reverse voltage and I want the motor to run in reverse at times as well. So I added a DPDT switch after the PWM and before the motor and this works perfect. My question is, will reversing the voltage hurt the motor? Short test indicate that it seems to work fine, but not sure what might happen with longer run times.

Usually not. The only issue that I ever recalled on a DC motor is non- symmetric brushes on the motor caused unusual wear when in reverse. If you are only doing it occasionally on one of these motors (rare), even then it will not be an issue. You should be fine.

 

Usually not. The only issue that I ever recalled on a DC motor is non- symmetric brushes on the motor caused unusual wear when in reverse. If you are only doing it occasionally on one of these motors (rare), even then it will not be an issue. You should be fine.

Great. Thanks!

 

Hello,

Newbie here. Need opinions on the best way to control the speed of a 3V dc motor. I have tried a couple of pots but the control is to condensed...in other words all the control is happening within a quarter turn. The range of RPMs I am looking for is 15-40, so pretty slow. The motor I have is Radio Shack's # 2730223

This unit looks to do exactly what I want but it looks like a servo in the beginning of the video. Should I go that route? Or do I keep the motor I have and use gears to knock down the RPMs? Pretty confused.

In the end, I'd like this to be battery powered and fit in a small project box.

 

DC motors speed is generally proportional to applied voltage. (A simple control approach is to use adjustable Buck regulator)
However the voltage drop across the winging resistance (and brushes if used) makes the motor slow under load. If the speed is to be maintained across varying load conditions - there are two classical approaches:
1. Using speed control loop based for instance on Tacho generators as feedback device (The buck feedback would be not output voltage but speed feedback)
2. Making the powering source feature negative output resistance, matching the one of the motor. In this way the voltage regulator will increase its output voltage by as much as the voltage drop in the motor winding...resulting in "idealized motor" running at constant speed. This was used in early days of Walkman. Implementing this approach requires positive current feedback (in addition to the common voltage feedback), but nevertheless is simpler than it sounds, especially for low power motors, where an OP-AMP and emitter follower could do. The follower just increases the current handling capabilities of the OA. The (-) input of the OA is used with a with negative voltage feedback from the emitter follower output. The motor is powered via a small current sensing resistor, which motor facing end is fed via resistor to the (+) input of the OA, along with the speed command via another resistor. The general scale factor (rpm/V) is defined by the negative voltage feedback divider.
The positive current feedback adds to the command at the OA (+) input pin, resulting in command rising with load current. A normal output resistance results in voltage dropping; a negative output resistance in its rising-(hence the term). The extent of voltage rising per A load is defined by the divider in the OA (+) pin i.e. by the resistors (current sense, positive feedback and command)

 

LEGACY THREAD.
Max.

 

@Plamen

Welcome to AAC. In the spirit of welcoming, I’d like to mention that you have fallen into a common new member trap. There are many excellent threads on the site, but many were posted years ago.

Responding to those is called “necroposting” (raising from the dead). Necroposting is not looked upon with favor.

FYI! I look forward to seeing your contributions to the community. On recent posts